1. Technical Field
The present invention relates to a package for electronic component and a piezoelectric resonator, in particular to a package for electronic component with an improved mounting terminal structure which enhances connection strength between the package and a circuit substrate, and to a piezoelectric resonator using the package.
2. Related Art
Piezoelectric resonators, in particular surface-mount piezoelectric quartz crystal resonators, are used as a reference frequency source in a wide range of apparatuses from communication devices to household appliances because of their smallness in size and ability to produce highly precise and stable frequencies with little aging change. Recently, as these apparatuses are becoming smaller and lighter, there are growing demands for even smaller surface-mount quartz crystal resonators.
As is generally known, a surface-mount quartz crystal resonator, which includes metal films (excitation electrodes) provided on both principal surfaces of a quartz crystal plate by vacuum deposition or sputtering, is housed in a package body, and the periphery of the package body is sealed hermetically with a lid by seam welding.
When using the quartz crystal resonator in in-vehicle equipment, the connection strength between the resonator and a circuit substrate becomes particularly important. This is because in-vehicle equipment is exposed to a harsh environment of low-to-high temperatures, and, if the line expansion coefficient of the package of the piezoelectric resonator differs from that of the circuit substrate having the piezoelectric resonator, solder for connecting the package to the circuit substrate may become distorted due to recurring temperature changes and may therefore suffer fatigue breaking. Also, along with downsizing in the shape of the surface-mount quartz crystal resonator, the area of the mounting terminals of the package has also been further downsized. One example of the surface-mount quartz crystal resonator with strengthened connection between the mounting terminals (excitation electrodes) and the circuit substrate is disclosed in JP-A-2005-108923. FIG. 9A is a sectional side surface diagram of the surface-mount quartz crystal resonator, and FIG. 9B is a bottom surface diagram of the resonator. A surface-mount quartz crystal resonator 70 includes: a plane rectangular package body 71 having a recess opened at the upper part of the package body 71, a quartz crystal resonator element 75 housed in this package body 71, and a lid 72 joined to the opened part at the upper part of the package body 71. The surface-mount quartz crystal resonator 70 is connected onto a wiring pattern 91 of a circuit substrate 90 with solder 85.
Referring to the bottom diagram of FIG. 9B, a pair of terminal electrodes 82, 83, which are provided along opposing sides of the bottom surface of the package body 71, include mutually opposing regions 82a, 83a and regions 82b, 83b where only one of the electrodes is not formed. These regions are point-symmetrically disposed with respect to a central point of the package body 71. Also, castellations C1 to C4 are provided at four corners of the package body 71. The castellations C1, C3 are coupled to the terminal electrodes 82, 83, respectively. Even if a stress is generated in the terminal electrodes 82, 83 because of the thermal expansion difference between the package body 71 and the circuit substrate 90 caused by changes in the temperature environment, the package body 71 having the described structure allows this stress to interactively escape toward the corner regions 82b, 83b where one of the terminal electrodes is not formed. This produces a stress planarly rotating the package around the central point of the package body 71 and thereby reduces the stress. As a result, it is disclosed that the occurrence of problems such as solder cracking is drastically suppressed.
Also, JP-A-2006-186667 discloses a package including four terminal electrodes at four corners of the rear surface of the package, each terminal electrode having a small projection in the center. It is disclosed that, by mounting the piezoelectric resonator on a circuit substrate employing this package, gaps are generated between the terminal electrodes and the circuit substrate. It is stated that, because these gaps are filled with solder in sufficient amount and thickness, sufficient connection strength can be achieved.
However, in terms of the package body 71 disclosed in JP-A-2005-108923, the terminal electrodes 82, 83 are made conductive to the piezoelectric resonator housed in the package via the respective castellations C1, C3 provided at the corners of the package body. Therefore, if cracks occur to sections where the terminal electrodes 82, 83 are coupled to the castellations C1, C3, there is a problem that the quartz crystal resonator may not operate.
In terms of the technique of JP-A-2006-186667, although the amount of solder between a terminal electrode plane of the package and a land electrode plane of the circuit substrate may increase, it is a problem that the amount of solder is not necessarily sufficient at an area where the distortion is concentrated.